1. Field of the Invention
The present invention relates to a reproducing method and a reproducing apparatus for an optical recording medium having a recording layer of a photochromic material etc. which reacts in a photon mode.
2. Description of the Background Art
In recent years, deep study is made on application of a photochromic material to a reloadable optical recording medium. When such a photochromic material is irradiated with light of a prescribed wavelength, its molecular structure is changed by photochemical reaction. Further, optical characteristics such as absorbance and a refractive index are additionally changed with respect to light of a constant wavelength, while the changed molecular structure can return to the original state when the photochromic material is irradiated with light of another wavelength or heated. Due to such properties, the photochromic material can be applied to an optical recording medium. In such an optical recording medium, it is possible to record information by irradiating the medium with light of a specific wavelength thereby changing its molecular structure, while the recorded information can be reproduced by detecting resulting changes of its optical characteristics, particularly absorbance. In relation to such an optical recording medium of a photon mode, it is well known that its reproducing power is disadvantageously reduced when the recorded information is repeatedly reproduced. Since photochromic molecules react in a photon mode, such reaction of the molecules slightly progresses even if the information is reproduced at a relatively low power level, to reduce the reproducing power following repetitive reproduction. As to a possible solution for such a problem, two methods are now under study.
The first method is adapted to reproduce information by detecting changes of optical characteristics other than absorbance, i.e., optical rotatory power, refractive index, birefringence etc., following photochromic reaction, with light of a wavelength allowing no absorption by the photochromic material. The second method is directed to development of the so-called gate type photochromic material which reacts only when a magnetic field, an electric field, heat or the like is applied thereto in addition to light.
However, it is difficult to apply the first method to multiplex recording which is prevailing means for high densification, for example. As to the second method, on the other hand, no success has yet been attained in synthesis of a gate type photochromic material which is responsive to a magnetic or electric field. Further, a heat gate type photochromic material loses high resolution, which characterizes the photon mode, by thermal diffusion.